Complex tessellations, extraordinary materials

An international team of researchers lead by the Physics Department at TUM
has discovered a reaction path that produces exotic layers with
semiregular structures. These kinds of materials are interesting because they
frequently possess extraordinary properties. In the process, simple organic
molecules are converted to larger units which form the complex, semiregular
patterns.

Only a few basic geometric shapes lend themselves to covering a surface
without overlaps or gaps using uniformly shaped tiles: triangles,
rectangles and hexagons. Considerably more and significantly more
complex regular patterns are possible with two or more tile shapes.
These are so-called Archimedean tessellations or tilings.

Materials can also exhibit tiling characteristics. These structures are
often associated with very special properties, for example unusual
electrical conductivity, special light reflectivity or extreme
mechanical strength. But, producing such materials is difficult. It
requires large molecular building blocks that are not compatible with
traditional manufacturing processes.

Complex tessellations through self-organization

The new building block (left, red outline) comprises two modified starting molecules connected to each other by a silver atom (blue).
This leads to the formation of complex, semiregular tessellations.
– Image: Klappenberger and Zhang / TUM

An international team led by Professors Florian Klappenberger and
Johannes Barth at the Chair of Experimental Physics of TUM, as well as
Professor Mario Ruben at the Karlsruhe Institute of Technology, have now
made a breakthrough in a class of supramolecular networks: They got
organic molecules to combine into larger building blocks with a complex
tiling formed in a self-organized manner.

As a starting compound, they used ethynyl iodophenanthrene, an easy to
handle organic molecule comprising three coupled carbon rings with an
iodine and an alkyne end. On a silver substrate, this molecule forms a
regular network with large hexagonal meshes.

Heat treatment then sets a series of chemical processes in motion,
producing a novel, significantly larger building block which then forms
a complex layer with small hexagonal, rectangular and triangular pores
virtually automatically and self-organized. In the language of geometry
this pattern is referred to as a semiregular 3.4.6.4 tessellation.

Atom economy through by-product recycling

“The scanning tunnel microscopy measurements we conducted at TUM show
clearly that the molecular reorganization involves many reactions that
would normally result in numerous by-products. In this case, however,
the by-products are recycled, meaning that the overall process runs with
great economy of atoms – nearly one hundred percent recovery – to arrive
at the desired end-product,” explains Prof. Klappenberger.

The researchers uncovered precisely how this happens in further
experiments. “Using X-ray spectroscopy measurements at the electron
storage ring BESSY II of the Helmholtz-Zentrum Berlin, we were able to
decipher how iodine splits from the starting product, hydrogen atoms
move to new positions and the alkyne groups capture the silver atom,”
explains lead author Yi-Qi Zhang.

By way of the silver atom, two starting building blocks bind together to
a new, larger building block. These new building blocks then form the
observed complex pore structure.

“We have discovered a completely new approach to produce complex
materials from simple organic building blocks,” summarizes
Klappenberger. “This is important for the ability to synthesize
materials with specific novel and extreme characteristics. These results
also contribute to better understanding the spontaneous appearance
(emergence) of complexity in chemical and biological systems.”

The research was funded by the German Research Foundation (in the
context of the Excellence Cluster Munich Center for Advanced Photonics,
as well as the priority program SPP 1459, the Transregio TR 88 3MET C5
and the DFG project KL 2294/3) and the European Research Council (ERC
Advanced Grant MolArt). Synthesis and characterization of the molecules
was done in the Karlsruhe Nano Micro Facility.